Glossy inks have various uses such as decoration and specular surfaces, and metal-containing ink compositions using silver particles or an aluminum paste as a filler are known. When the ink compositions are applied to substrates and an operation such as heating is performed thereon, substrates with a surface having a metallic layer formed thereon are obtained.
However, the metallic layers formed from the ink compositions have poorer glossiness and lower specularity than actual metals. It is thought that one reason for this is that the contact between metal atoms is insufficient in the formed metallic layers. As described above, even when using an ink composition containing a desired metal, it is unusual to form a metallic layer having desired glossiness and specularity and it is strongly desirable to develop an ink composition which can form a desired high-quality metallic layer.
Among various metals, metallic silver is widely used as a material of recording mediums and printing plates, and as a high-conductive material due to its superior conductive property. Examples of general conventional methods of manufacturing metallic silver include a method of heating silver oxide, which is an inorganic material, in the presence of a reductant. Specifically, for example, a paste may be prepared by dispersing granular silver oxide in a binder and adding a reductant thereto, and the paste may be applied to a substrate or the like and heated. In this manner, since heating is performed in the presence of the reductant, the silver oxide is reduced, the resulting metallic silver formed by the reduction is mutually fused, and a film including the metallic silver is formed.
However, when silver oxide is used as a material for forming metallic silver, a reductant is needed and there is a problem in that a treatment temperature thereof is extremely high, e.g., about 300° C. Furthermore, when metallic silver is used as a conductive material, in order to reduce the resistance of a formed film, it is necessary to use smaller silver oxide particles.
Regarding this, in recent years, a method of forming metallic silver using organic acid silver salt in place of an inorganic material such as silver oxide has also been reported. As the organic acid silver salt, for example, silver behenate has been reported (Patent Document 1). In addition, silver stearate and silver α-ketocarboxylate have been reported as new materials for forming metallic silver (Patent Documents 2 and 3).
However, even when using silver behenate, heating in the presence of a reductant is needed in order to form metallic silver. In addition, even when using silver stearate or silver α-ketocarboxylate, although the temperature is lower than in the case of the inorganic material, heating at about 210° C. or higher is needed in order to rapidly decompose the silver stearate or silver α-ketocarboxylate.
Therefore, a method of using silver β-ketocarboxylate, as a material for rapidly forming metallic silver even at low temperature without the need for a reductant, in place of silver oxide or organic acid silver salt has been disclosed (see Patent Documents 4 and 5). This method discloses that silver β-ketocarboxylate is used in combination with a compound having a lone pair such as an amine compound, a thiol compound or a phosphorus compound to improve stability of a silver ink composition in which the silver β-ketocarboxylate and the compound are blended, and metallic silver having a superior conductive property can be formed at a lower temperature than in the conventional cases.